Christian and Jewish catacombs share features that link their development, according to Rutgers. "They are quite similar in terms of architecture," he said. "If you're in a site where there are no inscriptions and no wall paintings, then it's hard to say whether it's Jewish or Christian."

There are no known writings that suggest that Christians adopted the practice of catacomb burials from Roman Jews. But the new dating evidence helps establish a chronology that supports this argument, Rutgers says.

"It's not unlikely that the Christians said to themselves, Well this is a great idea, let's copy it," he said.

Some scholars argue that the Christian catacombs have even earlier origins that tie in with pagan funerary customs.

Amanda Claridge, a classical archaeologist at the University of London, says it's unlikely the practice began with the Jews. "I think it evolved from the cremation burials [of pagan Romans], which were all rock-cut, underground depositories that date from the very early first century B.C.," she said.

Claridge adds that Roman pagans later switched from cremation burials to corpse burials, which necessitated bigger chambers like those found in the catacombs.

Roman Motifs

The Christians adopted traditional Roman motifs for their tombs, including garlands, flowers, birds, and other animals, according to Claridge.

"They get taken over into the Christian world and acquire ever more exclusively Christian associations," she said. "For instance, sheep are considered very Christian, but they are already there in the existing [ancient Roman] repertoire for the decoration of tombs."

The archaeologist says the catacombs were a cheap option for Rome's poorer people, who couldn't afford to buy a burial plot. "The Christians often belonged to that social-economic group," she added.

Given that Christianity grew out of Judaism, it might seem a fair assumption that Jewish customs also influenced early Christian burials. Yet ever since catacomb studies began in the 16th and 17th centuries, Christian scholars appear to have overlooked the Jewish connection.

The reason for this is theological, according to Rutgers, the lead study author.

"These scholars weren't interested in Judaism," he said. "It may sound very silly today, but they didn't like the idea that Christianity had Jewish roots. Therefore, they thought it wise not to investigate.

Even in the 20th century catacomb archaeology has clung to very old methods. It's a very traditional field, and this strong theological influence is still felt."

Claridge, of the University of London, added, "The early history of the exploration of the catacombs was done by members of [17th-century] society in Catholic Rome, where Jews were marginalized and often treated extremely badly."

"Since the 17th century it's been traditional that catacomb archaeology is done by members of the Catholic Church and nobody else," she said. "After all, the Church owns most of the catacombs of Rome."